The Human Colon in Evolution: Part 2, Fiber Foolishness

Submitted by Melissa on Wed, 05/18/2011 - 18:37

Suggestions that humans may have obtained more calories from SCFA in the past are rooted in estimates of fiber consumption from the Paleolithic. Evidence is rather sparse and limited to coprolites. In the burgeoning field of evolutionary medicine, anthropologists have become very interested in the Paleolithic diet and its relevance for promoting health today. Some of the landmark papers in the field have cited these coprolite studies as evidence for fiber intakes as high as 150 grams as day, well over what any known human culture currently consumes (M. Konner & S Boyd Eaton, 2010). Even if the method for estimating fiber consumption from coprolites is accurate, they may not support the conclusion that they represent some species level optimal and may in fact suggest a ceiling for safe fiber consumption that great apes do not share.

The argument for studying the Paleolithic diet in order to see what is optimal for modern humans to eat stems from the fact that archeological remains suggest that Paleolithic hunter-gatherers were healthier than their agrarian descendents (S B Eaton, M. J. Konner, & Shostak, 1996). Generally, they were taller, had fewer skeletal pathologies, and their teeth were in much better condition 1. One of the sources for these high fiber estimates is coprolites from prehistoric Indians in the North American desert southwest, who consumed as much as 150 to 225 grams of fiber a day (Leach & Sobolik, 2010). Far from being in admirable health, evidence shows they had extensive dental caries. Some have speculated that this was caused by high levels of phytoliths in their diet, which wore down their teeth (Danielson & Reinhard, 1998). But other populations with extensive tooth wear do not exhibit high levels of caries2 (B. H. Smith, 1984). This raises the question of whether or not wear really caused their caries or if perhaps their fiber consumption caused them.

Clues come from modern nutritional biochemistry. Dietary fiber has the ability to reduce blood levels of Vitamin D, which is vital in tooth and bone mineralization (Batchelor & Compston, 1983). This may be the reason that some populations with high-fiber diets in Asia exhibit vitamin D deficiency despite adequate sun exposure. Children on macrobiotic diets, which are high in fiber, have higher than normal rates of rickets (Dagnelie et al., 1990). However, macrobiotic diets and those of rural Asians are notably low in animal products and high in plants different from those our Paleolithic ancestors ate, which contain mineral-leaching phytic acid (Raboy, 2001). The fiber in other primate diets and presumably in Paleolithic diets is mostly dicot vegetable fiber, whereas modern grain fibers come from monocotyledonous plants (Milton, 1989). It’s also possible that these problems would not occur at Paleolithic levels of animal product consumption, as animal products are rich sources of vitamin D and minerals (M. Konner & S Boyd Eaton, 2010).

Other anthropologists have tried to infer ancestral fiber consumption based on the diets of modern foraging populations and agrarians, but these have run into their own problems. Incorrect laboratory methods of analysis marred early data sets, though some of these data sets are still being cited in newer papers. Some examples include an estimate for Ugandan fiber consumption of 150 grams a day that was revised to 70 grams a day and an estimate for Kenyans of 130 grams a day reduced to 86 grams a day (Wrangham, Conklin-Brittain, & C. C. Smith, 2002). Other problems have come from analyzing fiber outside of dietary context. For example, much like we don’t consume the peels of bananas, Hadza don’t consume whole wild tubers. When they eat tubers, they chew them and the excess fiber is spit out. Obviously estimates of fiber consumption based on the whole tuber are overestimates (Schoeninger, 2001).

Fiber spit out by the Hadza

Given this, the use of extremely high estimates for Paleolithic fiber intake based on limited data as a baseline for optimal consumption seems misplaced. No known culture consumes over 100 grams of fiber. The highest recent estimate was 86 grams for some agrarian cultures in Africa (Wrangham et al., 2002).

Some of the issue is also overemphasis on fiber, when other food constituents that play a similar role may have been more important in human evolution. Early optimism that high fiber could prevent many diseases of civilization like heart disease and type II diabetes spurred many studies on the matter. These have had mixed results. There have been several expensive failed studies such as the forty-nine thousand women Dietary Modification Trial of the Women’s Health Initiative which found that increasing dietary fiber had no effect on risk of colon cancer, breast cancer, or heart disease and no effect on weight loss (Beresford et al., 2006). Those who cling to the fiber hypothesis insist that the trials have not been long enough or high enough in fiber (Byers, 2000).

Focus on fiber in the past was on its abilities as indigestible bulking matter to increase digestive transit time and bind up certain food constituents. (J. Smith, Yokoyama, & German, 1998). The dominant theory was that slower transit time allowed carcinogens and other potential toxins to fester in the body. This idea that spawned a cottage industry of quacks and religious movements advertising “cleanses” (Kellogg, 1923) that has remained robust to this day , but has not stood up to scientific scrutiny.

Next up: fiber or bacteria??

1. This argument seems suspect, since while early agrarians seemed to have had high levels of disease based on skeletal evidence, later agrarians and pastoralists are often much taller than people in the Paleolithic and also exhibit low incidence of pathology.